1. Field of the Invention
This invention relates to a misfire-detecting system for internal combustion engines, and more particularly to a misfire-detecting system which detects a misfire attributable to a fuel supply system of the engine.
2. Prior Art
In an internal combustion engine having spark plugs, a misfire can occur, in which normal ignition does not take place at one or more of the spark plugs. Misfires are largely classified into ones attributable to the fuel supply system and ones attributable to the ignition system. Misfires attributable to the fuel supply system are caused by the supply of a lean mixture or a rich mixture to the engine, while misfires attributable to the ignition system are caused by failure to spark (so-called mis-sparking), i.e. normal spark discharge does not take place at the spark plug, for example, due to smoking of the spark plug or wetting thereof with fuel or abnormality in the sparking voltage supply system.
The present assignee has already proposed a misfire-detecting system for detecting misfires attributable to the fuel supply system, which comprises sparking voltage-detecting means which detects sparking voltage, i.e. voltage across electrodes of the spark plug, based upon an ignition command signal, and misfire-determining means which determines that a misfire has occurred based on the detected value of the sparking voltage, e.g. when a time period over which the detected value of the sparking voltage exceeds a predetermined reference value (Japanese Patent Application No. 3-326507 and corresponding U.S. Ser. No. 07/846,238 filed Mar. 5, 1992).
Further, the present assignee has proposed a misfire-detecting system which is an improved version of the above-mentioned misfire-detecting system, which comprises recharging means for recharging the ignition coil to again generate high voltage across the spark plug after generation of the ignition command signal, measuring means for measuring by a counter a time period over which the sparking voltage exceeds a predetermined time period immediately after the recharging, and misfire-determining means for determining that a misfire has occurred in the engine if the measured time period exceeds a predetermined time period (Japanese Patent Application No. 4-93775 filed Mar. 19, 1992).
However, this proposed misfire-detecting system has the disadvantage that when dielectric breakdown of a mixture supplied to the cylinder occurs due to the recharging, it can happen that the time period over which the sparking voltage exceeds the predetermined voltage, does not exceed the predetermined time period even when a misfire has occurred, resulting in a misjudgment on the occurrence of a misfire.